The RCX-I2C module is interface between your RCX and any I2C (also known as IIC or I?C) devices like SRF08 or SRF10 distance sensors. You can send commands from RCX to RCX-I2C module using RCX IR port and receive data back to RCX. There are nine commands which can be sent from RCX to RCX-I2C module which control operation of this module. Documentation with NQC example program and header file is available to get started with using your I2C device or sensor. There is an I2C connector on the module, and you can connect upto 128 I2C devices to this I2C module (that's a standard feature of I2C bus). Bit rate on the I2C bus is approximately 100KHz.

The bottom side of this RCX-I2C module contains IR sensor and transmitter, which needs to be mounted roughly in sight of RCX IR port. This picture shows how you could mount this RCX-I2C module on RCX.
The RCX-I2C module responds to commands sent to it over the IR link from the RCX.

Standard LEGO electrical connector can be used to supply the power to this RCX-I2C module.

When connected to RCX, use motor power setting to be "max power" and "forward".

Subsequently RCX-I2C uses the same power source to supply power to your I2C devices (5 volts, up to 200 mA).

While communicating on IR interface use 'LOW Range' setting on your RCX.

3) Byte Read: Read the "N" no or byte from "IIC Device" starting at "Register "
0x55 0xff 0xaa 0x52 < IIC Device > < Register > < N no of bytes >
byte data is transferred the designated variable in RCX set using command Set Pointer
Return value is no of bytes left in the RCX-I2C internal buffer as standard RCX message.
to access the Next byte send message "78" to RCX-I2C
to abort the transfer send message "255" to RCX-I2C
you need to either read entire buffer or issue abort message to RCXI2C before using any other command

4) Integer Read: Read the "N" no or Integers from "IIC Device" starting at "Register "
0x55 0xff 0xaa 0x52 < IIC Device > < Register > < N no of Integers LOW:HIGH >
integer data is transferred the designated variable in RCX set using command Set Pointer
Return value is no of integers left in the RCX-I2C internal buffer as standard RCX message.
to access the Next integer send message "78" to RCX-I2C
to abort the transfer send message "255" to RCX-I2C
you need to either read entire buffer or issue abort message to RCX-I2C before using any other command

5) Alive:
0x55 0xff 0xaa 0x41
checks if RCX-I2C is alive and ready
returns the standard RCX message of "66" if found